In general, any equation like
(assuming
) is solved by

So, in your case, the solution is

Dividing by a fraction means to multiply by the inverse of that fraction:

Answer:
35 different selections of soundtracks are possible
Step-by-step explanation:
Total number of soundtracks available at the store = 7
Enny decides to purchase 3 movie soundtracks.
So, in this problem we have to find in how many ways can Enny chose 3 soundtracks out of 7. The order in which the soundtracks are chosen does not matter e.g soundtracks A,B and C are the same as soundtracks B,C and A.
In an arrangement when the order does not matter, we use combinations to find the total possible number of arrangements or selections.
So, for this problem choosing 3 soundtracks out of 7 means forming combinations from 7 objects taken 3 at a time. This can be represented mathematically as:

The general formula for combinations is:

Using the values, we get:

This means, Enny can make 35 different selections of soundtracks.
4 out of 32 or 4/32 are infested with beetles, you know because it shows what beetles look like and it shows 4 trees infected with beetles out of the possible 32 trees